Technical Abstract:
It is hypothesized that plant hyperaccumulation of Ni evolved as a defense mechanism against diseases and insects. Two hyperaccumulators, Alyssum corsicum and Berkheya coddii, were compared to cabbage (Brassica oleracea) grown in MES-HEPES buffered nutrient solutions and maintained at four pH levels (5.6, 6.2, 6.8 and 7.5) and two Ni concentrations (31.6 M and 316 M). This experiment followed an earlier pot experiment comparing a Brockman serpentine soil with two Ni-smelter contaminated soils where soil specific ion interactions played a significant role in determining the effect of soil pH on Ni hyperaccumulation. This study was designed to characterize the effect of solution pH on plant Ni uptake independent of soil ionic interactions. A. corsicum and B. coddii showed hyperaccumulation of Ni in shoots increased with solution pH. B. oleracea showed signs of Ni-phytotoxicity stress at all pH levels except pH 7.5 where Ni accumulation was significantly reduced. The serpentine soils where these species evolved are usually pH neutral which strongly reduces soil solution Ni levels, yet they accumulate Ni at levels that provide defense against pests. A. corsicum may have evolved a unique mechanism to accumulate high shoot Ni at increasingly alkaline pH levels despite reduction of soil Ni solubility with increasing pH. These findings suggest that A. corsicum's root membrane uptake carrier optimum is at higher pH levels than other species. This finding suggests an important component of Ni hyperaccumulation allowed this species to accumulate soil Ni at pH levels which occur naturally where the crop evolved, a pattern very different from crop plants.